Moreover older models which have become slow from updating to the latest (installable) iOS version and/or those with full memory and/or those with failing batteries.

The first easy fix that you must try is hard rebooting by pressing and holding the power button + home button (power button + volume down button for iPhones 7/7+) until the phone restarts. Charging it if you feel the battery was drained, then trying to reboot again.

Or if you feel comfortable tinkering with your iDevice and have experience with it, you can disconnect the battery, press and hold the power button for a few seconds, reconnect the battery and try again to boot.

Letting your battery drain has the same "reset effect" which is why it eventually woke up.

Resources

Repairability

Stay in the loop

It's time to speak out for your right to repair

We have a chance to guarantee our right to repair electronic
equipment—like smartphones, computers, and even farm equipment. This is a once-in-a-generation
chance to protect local repair jobs—the corner mom-and-pop repair shops that
keep getting squeezed out by manufacturers.

Join the cause and tell your state representative to support Right to Repair. Tell them you believe repair should be fair, affordable, and accessible. Stand up for your right to repair!